Interactions between schistosomes and their snail hosts play an important role in the evolutionary development of the host and parasite genetic systems. Gene flow, genetic drift and selection all potentially act on host-parasite interactions, driving co-evolutionary trajectories and dictating disease dynamics. Assessing the degree and structure of genetic heterogeneity of both participants in a host-parasite interaction is an essential first step in gaining predictive power over epidemiology. The recent characterization of microsatellite markers for Schistosoma mansoni and Biomphalaria glabrata provides an opportunity to study population structure of both host and parasite on a local spatial scale, using markers that exhibit allelism. Specific aims include: 1) Determine the relative nature of population subdivision for schistosomes and their snail host populations based on allelic data from microsatellites, and assess how that genetic variation changes on both a spatial and temporal scale; and 2) Correlate microsatellite allelic heterogeneity with biomarkers for schistosome virulence and snail resistance to assess the relationship between genetic variation and phenotypic characters of epidemiological importance. Knowledge of the distribution of host and parasite genetic diversity within and among individual hosts, and within and among populations, will allow us to estimate parasite gene flow, assess the importance of multiple genotypes on parasite virulence, and begin to understand the interplay of micro-evolutionary forces that drive disease dynamics. Results will yield evolutionary insights into the epidemiological process, help identify genetic consequences of control strategies, and complement concurrent immuno-epidemiology studies of humans in endemic Brazilian communities.